U.S. patent number 7,629,593 [Application Number 11/819,707] was granted by the patent office on 2009-12-08 for lithographic apparatus, radiation system, device manufacturing method, and radiation generating method.
This patent grant is currently assigned to ASML Netherlands B.V.. Invention is credited to Vadim Yevgenyevich Banine, Edwin Johan Buis, Wouter Anthon Soer, Tjarko Adriaan Rudolf Van Empel, Maarten Marinus Johannes Wilhelmus Van Herpen.
United States Patent |
7,629,593 |
Buis , et al. |
December 8, 2009 |
Lithographic apparatus, radiation system, device manufacturing
method, and radiation generating method
Abstract
A lithographic apparatus includes a radiation system constructed
to provide a beam of radiation from radiation emitted by a
radiation source. The radiation system includes a contaminant trap
configured to trap material emanating from the radiation source.
The contaminant trap includes a contaminant engaging surface
arranged in the path of the radiation beam that receives the
material emanating from the radiation source during propagation of
the radiation beam in the radiation system. The radiation system
also includes a liquid tin cooling system constructed to cooling
the contaminant trap with liquid tin. The apparatus includes an
illumination system configured to condition the radiation beam, a
support constructed to support a patterning device configured to
impart the radiation beam with a pattern in its cross-section, a
substrate table constructed to hold a substrate, and a projection
system configured to project the patterned radiation beam onto a
target portion of the substrate.
Inventors: |
Buis; Edwin Johan (Belfeld,
NL), Banine; Vadim Yevgenyevich (Helmond,
NL), Van Empel; Tjarko Adriaan Rudolf (Eindhoven,
NL), Van Herpen; Maarten Marinus Johannes Wilhelmus
(Heesch, NL), Soer; Wouter Anthon (Nijmegen,
NL) |
Assignee: |
ASML Netherlands B.V.
(Veldhoven, NL)
|
Family
ID: |
40159240 |
Appl.
No.: |
11/819,707 |
Filed: |
June 28, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090001288 A1 |
Jan 1, 2009 |
|
Current U.S.
Class: |
250/492.1;
250/492.2; 250/492.22; 250/493.1; 250/504R; 355/30 |
Current CPC
Class: |
G03F
7/70916 (20130101) |
Current International
Class: |
G21K
5/04 (20060101) |
Field of
Search: |
;250/492.1,492.2,492.21,492.22,492.3,493.1,504R,492.23,505.1
;165/80.4 ;355/30,34,53,71 ;378/34,119 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Berman; Jack I
Assistant Examiner: Rausch; Nicole Ippolito
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman
LLP
Claims
What is claimed is:
1. A lithographic apparatus comprising: a radiation system
constructed to provide a beam of radiation from radiation emitted
by a radiation source, the radiation system comprising a
contaminant trap configured to trap material emanating from the
radiation source, the contaminant trap comprising a static part and
a rotating part, the rotating part comprising a contaminant
engaging surface arranged in the path of the radiation beam that
receives the material emanating from the radiation source during
propagation of the radiation beam in the radiation system, and a
liquid tin cooling system constructed to cool the contaminant trap
with liquid tin; an illumination system configured to condition the
radiation beam; a support constructed to support a patterning
device, the patterning device being configured to impart the
radiation beam with a pattern in its cross-section to form a
patterned radiation beam; a substrate table constructed to hold a
substrate; and a projection system configured to project the
patterned radiation beam onto a target portion of the
substrate.
2. An apparatus according to claim 1, wherein the liquid tin
cooling system is arranged to condition the temperature of the
contaminant trap.
3. An apparatus according to claim 1, wherein the liquid tin
cooling system comprises a closed liquid tin circuit arranged
inside the static part of the contaminant trap.
4. An apparatus according to claim 3, wherein the liquid tin
cooling system comprises a liquid tin supply channel inside the
static part of the contaminant trap, the supply channel extending
to the rotating part of the contaminant trap for supplying the
liquid tin towards an external surface of the rotating part, the
external surface comprising the contaminant engaging surface.
5. An apparatus according to claim 4, wherein the contaminant
engaging surface is disposed on a foil, and wherein the liquid tin
cooling system further comprises a return path along a leading edge
of the foil comprised in the rotating part of the contaminant
trap.
6. An apparatus according to claim 4, wherein the contaminant
engaging surface is disposed on a foil, and wherein the liquid tin
cooling system further comprises a return path embedded in the foil
comprised in the rotating part of the contaminant trap.
7. An apparatus according to claim 1, wherein the liquid tin
cooling system comprises a semi-open liquid tin circuit constructed
to directly cool the rotating part of the contaminant trap.
8. An apparatus according to claim 1, wherein the liquid tin
cooling system comprises an exterior supply channel having a spray
end arranged to spray the rotating part of the contaminant
trap.
9. An apparatus according to claim 8, wherein the contaminant
engaging surface is disposed on a foil, and wherein the spray end
is arranged near the foil comprised in the rotating part of the
contaminant trap.
10. An apparatus according to claim 1, further comprising a gas
inlet and a heating element, both arranged near the contaminant
trap.
11. An apparatus according to claim 1, further comprising a radical
or plasma generating unit.
12. An apparatus according to claim 1, wherein an exterior surface
of the contaminant trap comprises a top layer having a low
oxidation rate.
13. An apparatus according to claim 12, wherein the top layer
comprises gold.
14. An apparatus according to claim 1, wherein the contaminant
engaging surface is disposed on a foil, the foil having a segment
that is substantially porous, and wherein a liquid tin supply
channel ends in the porous segment of the foil.
15. A radiation system constructed to provide a beam of radiation
from radiation emitted by a radiation source, the radiation system
comprising: a contaminant trap configured to trap material
emanating from the radiation source, the contaminant trap
comprising a static part and a rotating part, the rotating part
comprising a contaminant engaging surface arranged in the path of
the radiation beam that receives the material emanating from the
radiation source during propagation of the radiation beam in the
radiation system; and a liquid tin cooling system constructed to
cool the contaminant trap with liquid tin.
16. A device manufacturing method comprising: trapping material
emanating from a radiation source using a contaminant trap
comprising a static part and a rotating part, the rotating part
comprising a contaminant engaging surface by arranging the surface
in a radiation beam emitted by the radiation source; cooling the
contaminant trap with liquid tin; conditioning the radiation beam;
imparting the radiation beam with a pattern in its cross-section
using a patterning device to form a patterned radiation beam; and
projecting the patterned radiation beam onto a target portion of a
substrate.
17. A method according to claim 16, further comprising collecting
liquid tin that is dropped into a chamber in which the contaminant
trap is arranged, and reusing collected liquid tin to cool the
contaminant trap.
18. A method according to claim 16, further comprising regenerating
tin liquid in a cooling circuit of a liquid tin cooling system.
19. A method according to claim 16, further comprising pre-treating
a contaminant trap exterior surface for improved surface wetting
characteristics.
20. A method according to claim 19, wherein said pre-treating
comprises heating the exterior surface.
21. A method according to claim 20, wherein said heating the
exterior surface is performed in a hydrogen atmosphere.
22. A method according to claim 19, wherein said pre-treating
comprises introducing radicals or a plasma near the contaminant
trap.
23. A method according to claim 22, wherein said plasma is an
oxygen plasma.
24. A method according to claim 19, wherein said pre-treating
comprises coating the exterior surface with a top layer having a
low oxidation rate.
25. A method according to claim 24, wherein said top layer
comprises gold.
26. A method according to claim 24, wherein the top layer has a
solubility in liquid tin of less than about 0.05%.
27. A method according to claim 26, wherein the top layer has a
solubility in liquid tin of less than about 0.005%.
28. A radiation generating method comprising: trapping material
emanating from a radiation source using a contaminant trap
comprising a static part and a rotating part, the rotating part
comprising a contaminant engaging surface by arranging the surface
in a radiation beam emitted by the radiation source; and cooling
the contaminant trap with liquid tin.
Description
FIELD
The present invention relates to a lithographic apparatus, a
radiation system, a device manufacturing method, and a radiation
generating method.
BACKGROUND
A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. comprising part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti-parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
To image smaller features, it has been proposed to use extreme
ultraviolet radiation (EUV) with a wavelength in the range of 5-20
nanometers, in particular, 13.5 nanometers, or a charged particle
beam, e.g. an ion beam and an electron beam, as the exposure
radiation in a lithographic apparatus. These types of radiation
need the beam path in the apparatus to be evacuated to avoid
absorption. Since there are no known materials suitable to make a
refractive optical element for EUV radiation, EUV lithographic
apparatus use mirrors in the radiation, illumination and projection
systems. Such mirrors are highly susceptible to contamination,
thereby reducing their reflectivity and hence the throughput of the
apparatus. Further, sources for EUV may produce debris whose entry
into the illumination system should be minimized.
In order to reduce the chance of debris entering the illumination
system, contaminant traps may be used. Such traps are disposed in
the radiation system downstream of the source. The traps comprise
elements that provide a surface on which debris can deposit.
Conventional radiation systems may also comprise a collector which
collects the radiation beam. It has been found that debris may also
deposit on elements in the collector. The deposit of debris on the
collector significantly reduces its operational lifetime before it
must be cleaned.
It has been found that as the temperature of elements in the
contaminant trap increases, the greater the contamination, and
hence, the shorter the lifetime of the collector. This is because
it has been found that at higher temperatures, the elements of the
contaminant trap may become secondary sources of contamination. In
particular, certain debris on the elements may be vaporized. The
vaporized debris then goes on to further contaminate the collector.
Further, in applying more powerful EUV sources causing increasing
heat loads, contaminant trap elements may melt and/or vaporize,
which may cause a collapse of the entire debris barrier.
SUMMARY
It is desirable to counteract the effects of a high temperature
realized by the contaminant trap.
According to an aspect, there is provided a lithographic apparatus
comprising a radiation system constructed to provide a beam of
radiation from radiation emitted by a radiation source. The
radiation system comprises a contaminant trap configured to trap
material emanating from the radiation source. The contaminant trap
comprises a contaminant engaging surface arranged in the path of
the radiation beam that receives the material emanating from the
radiation source during propagation of the radiation beam in the
radiation system, and a liquid tin cooling system constructed to
cool the contaminant trap with liquid tin. The apparatus also
comprises an illumination system configured to condition the
radiation beam, a support constructed to support a patterning
device, the patterning device being configured to impart the
radiation beam with a pattern in its cross-section to form a
patterned radiation beam, a substrate table constructed to hold a
substrate, and a projection system configured to project the
patterned radiation beam onto a target portion of the
substrate.
According to an aspect, there is provided a radiation system
constructed to provide a beam of radiation from radiation emitted
by a radiation source. The radiation system comprises a contaminant
trap configured to trap material emanating from the radiation
source. The contaminant trap comprises a contaminant engaging
surface arranged in the path of the radiation beam that receives
the material emanating from the radiation source during propagation
of the radiation beam in the radiation system. The radiation system
further comprises a liquid tin cooling system constructed to
cooling the contaminant trap with liquid tin.
According to an aspect, there is provided a device manufacturing
method trapping material emanating from a radiation source using a
contaminant trap comprising a contaminant engaging surface
arranging the surface in a radiation beam emitted by the radiation
source, cooling the contaminant trap with liquid tin, conditioning
the radiation beam, imparting the radiation beam with a pattern in
its cross-section using a patterning device to form a patterned
radiation beam, and projecting the patterned radiation beam onto a
target portion of a substrate.
According to an aspect, there is provided a radiation generating
method comprising trapping material emanating from a radiation
source using a contaminant trap comprising a contaminant engaging
surface by arranging the surface in a radiation beam emitted by the
radiation source, and cooling the contaminant trap with liquid
tin.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying schematic drawings
in which corresponding reference symbols indicate corresponding
parts, and in which:
FIG. 1 depicts a lithographic apparatus according to an embodiment
of the invention;
FIG. 2 depicts a schematic cross sectional view of a contaminant
trap comprised in a radiation system according to an embodiment of
the invention;
FIG. 3 depicts a schematic perspective view of the contaminant trap
comprised in a radiation system of FIG. 2;
FIG. 4 depicts a schematic cross sectional view of a contaminant
trap comprised in a radiation system according to an embodiment of
the invention;
FIG. 5 depicts a schematic cross sectional view of a contaminant
trap comprised in a radiation system according to an embodiment of
the invention;
FIG. 6 depicts a schematic cross sectional view of a radiation
system according to an embodiment of the invention;
FIG. 7 depicts a schematic cross sectional view of a radiation
system according to an embodiment of the invention;
FIG. 8 depicts a schematic cross sectional view of a section of a
radiation system according to an embodiment of the invention;
FIG. 9 depicts a view of a droplet on a platelet;
FIG. 10 depicts a view of a droplet on a platelet;
FIG. 11 depicts a view of a droplet on a platelet;
FIG. 12 depicts a view of a droplet on a platelet;
FIG. 13 depicts a view of a droplet on a platelet; and
FIG. 14 depicts a view of a droplet on a platelet.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a lithographic apparatus according to
one embodiment of the invention. The apparatus comprises: an
illumination system (illuminator) IL configured to condition a
radiation beam B (e.g. UV radiation or visible light radiation); a
support structure (e.g. a mask table) MT constructed to support a
patterning device (e.g. a mask) MA and connected to a first
positioner PM configured to accurately position the patterning
device in accordance with certain parameters; a substrate table
(e.g. a wafer table) WT constructed to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioner PW
configured to accurately position the substrate in accordance with
certain parameters; and a projection system (e.g. a refractive
projection lens system) PS configured to project a pattern imparted
to the radiation beam B by patterning device MA onto a target
portion C (e.g. comprising one or more dies) of the substrate
W.
The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, configured to direct,
shape, or control radiation.
The support structure supports, i.e. bears the weight of, the
patterning device. It holds the patterning device in a manner that
depends on the orientation of the patterning device, the design of
the lithographic apparatus, and other conditions, such as for
example whether or not the patterning device is held in a vacuum
environment. The support structure can use mechanical, vacuum,
electrostatic or other clamping techniques to hold the patterning
device. The support structure may be a frame or a table, for
example, which may be fixed or movable as desired. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device."
The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples
of patterning devices include masks, programmable mirror arrays,
and programmable LCD panels. Masks are well known in lithography,
and include mask types such as binary, alternating phase-shift, and
attenuated phase-shift, as well as various hybrid mask types. An
example of a programmable mirror array employs a matrix arrangement
of small mirrors, each of which can be individually tilted so as to
reflect an incoming radiation beam in different directions. The
tilted mirrors impart a pattern in a radiation beam which is
reflected by the mirror matrix.
The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system."
As here depicted, the apparatus is of a reflective type (e.g.
employing a reflective mask). Alternatively, the apparatus may be
of a transmissive type (e.g. employing a transmissive mask).
The lithographic apparatus may be of a type having two (dual stage)
or more substrate tables (and/or two or more mask tables). In such
"multiple stage" machines the additional tables may be used in
parallel, or preparatory steps may be carried out on one or more
tables while one or more other tables are being used for
exposure.
The lithographic apparatus may also be of a type wherein at least a
portion of the substrate may be covered by a liquid having a
relatively high refractive index, e.g. water, so as to fill a space
between the projection system and the substrate. An immersion
liquid may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the projection system.
Immersion techniques are well known in the art for increasing the
numerical aperture of projection systems. The term "immersion" as
used herein does not mean that a structure, such as a substrate,
must be submerged in liquid, but rather only means that liquid is
located between the projection system and the substrate during
exposure.
Referring to FIG. 1, the illuminator IL receives a radiation beam
from a radiation source SO. The source and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser. In such cases, the source is not considered to
form part of the lithographic apparatus and the radiation beam is
passed from the source SO to the illuminator IL with the aid of a
beam delivery system comprising, for example, suitable directing
mirrors and/or a beam expander. In other cases the source may be an
integral part of the lithographic apparatus, for example when the
source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system if desired, may be referred
to as a radiation system.
The illuminator IL may comprise an adjuster configured to adjust
the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may comprise various
other components, such as an integrator and a condenser. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
The radiation beam B is incident on the patterning device (e.g.,
mask MA), which is held on the support structure (e.g., mask table
MT), and is patterned by the patterning device. Having traversed
the mask MA, the radiation beam B passes through the projection
system PS, which focuses the beam onto a target portion C of the
substrate W. With the aid of the second positioner PW and position
sensor IF2 (e.g. an interferometric device, linear encoder or
capacitive sensor), the substrate table WT can be moved accurately,
e.g. so as to position different target portions C in the path of
the radiation beam B. Similarly, the first positioner PM and
another position sensor IF1 can be used to accurately position the
mask MA with respect to the path of the radiation beam B, e.g.
after mechanical retrieval from a mask library, or during a scan.
In general, movement of the mask table MT may be realized with the
aid of a long-stroke module (coarse positioning) and a short-stroke
module (fine positioning), which form part of the first positioner
PM. Similarly, movement of the substrate table WT may be realized
using a long-stroke module and a short-stroke module, which form
part of the second positioner PW. In the case of a stepper (as
opposed to a scanner) the mask table MT may be connected to a
short-stroke actuator only, or may be fixed. Mask MA and substrate
W may be aligned using mask alignment marks M1, M2 and substrate
alignment marks P1, P2. Although the substrate alignment marks as
illustrated occupy dedicated target portions, they may be located
in spaces between target portions (these are known as scribe-lane
alignment marks). Similarly, in situations in which more than one
die is provided on the mask MA, the mask alignment marks may be
located between the dies.
The depicted apparatus could be used in at least one of the
following modes:
1. In step mode, the mask table MT and the substrate table WT are
kept essentially stationary, while an entire pattern imparted to
the radiation beam is projected onto a target portion C at one time
(i.e. a single static exposure). The substrate table WT is then
shifted in the X and/or Y direction so that a different target
portion C can be exposed. In step mode, the maximum size of the
exposure field limits the size of the target portion C imaged in a
single static exposure.
2. In scan mode, the mask table MT and the substrate table WT are
scanned synchronously while a pattern imparted to the radiation
beam is projected onto a target portion C (i.e. a single dynamic
exposure). The velocity and direction of the substrate table WT
relative to the mask table MT may be determined by the
(de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
3. In another mode, the mask table MT is kept essentially
stationary holding a programmable patterning device, and the
substrate table WT is moved or scanned while a pattern imparted to
the radiation beam is projected onto a target portion C. In this
mode, generally a pulsed radiation source is employed and the
programmable patterning device is updated as desired after each
movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
Combinations and/or variations on the above described modes of use
or entirely different modes of use may also be employed.
FIG. 2 and depict a schematic cross sectional view and a schematic
perspective view, respectively, of a contaminant trap 10 comprised
in a radiation system according to an embodiment. The radiation
system is constructed to provide a beam of radiation from radiation
emitted by a radiation source SO (shown in FIG. 1). The radiation
source SO can be formed by a discharge plasma. The radiation source
SO can be of the EUV type and may be a tin (Sn) based plasma
source. Alternatively, the EUV type radiation source SO might use a
gas or vapour, such as Xe gas or Li vapor. The contaminant trap 10,
which may be considered to be a rotating foil trap, may comprise a
rotatable contaminant trap element 8 configured to trap material
emanating from the radiation source SO. Thereto, the rotatable
contaminant trap element 8 comprises multiple elements arranged in
the path of the radiation beam on which the material emanating from
the radiation source can deposit during propagation in the
radiation system. In the contaminant trap 10 shown in FIGS. 2 and
3, the multiple elements arranged in the path of the radiation beam
comprise metal platelets 9, also called foils. The foils or
platelets 9 comprise debris or contaminant receiving surfaces that
are arranged in the path of the radiation beam to prevent debris,
i.e. contaminant material, including particles, thrust by the
source from reaching optical components of the radiation system,
e.g. a collector and the illuminator IL. The foils are arranged
radially around a longitudinal axis O of the contaminant trap
10.
The contaminant trap 10 comprises a static part 1, and a plurality
of ring-shaped elements 2, 3, 4 and 5 arranged around the static
part 1 that support and guide a rotating part 6 on which the
rotatable contaminant trap element 8 with the foils 9 is built. The
foils 9 form strips that are preferably manufactured from metal,
e.g. molybdene. The foils 9 are sealingly connected to the rotating
part 6 via a ring-shaped seal 7. The rotating part 6 is driven by a
gas flow flowing from flow openings 11 in the exterior of the
static part 1. The gas also serves as a bearing between the static
part 1 and the rotating part 6. Further, the contaminant trap 10
comprises a channel structure 12, as will be explained in further
detail below.
The contaminant trap 10 further comprises a liquid tin cooling
system constructed to cool the contaminant trap 10, and especially
the rotatable contaminant trap element 8, with liquid tin. By
cooling the contaminant trap 10 with liquid tin, a contaminant trap
10 is obtained wherein the occurrence of a high temperature may be
counteracted. The liquid tin cooling may be considerably more
effective than heat transfer via radiation. It is noted that heat
conduction via gas particles in the radiation system is relatively
poor, due to the vacuum that is applied during operation. Also,
conduction via the material of the rotatable contaminant trap
element 8 is relatively small since the foils 9 are very thin and
material contact to the static part 1 is relatively small as the
rotating part 6 is supported via bearings. As a result, liquid tin
cooling may significantly improve the transfer of heat.
Consequently, the occurrence of high temperatures, e.g. up to and
even above 650.degree. C., may be reduced. Since such high
temperatures may be avoided, undesired melting processes of
elements in the radiation system may also be avoided. Further, by
cooling with liquid tin, radiation sources having a relatively
large power can be applied, e.g. up to circa 100 kW. By using a
liquid tin cooling system, also the frequency of the radiation
source and the running time of the apparatus can be relatively
high. It is further noted that the application of a liquid tin
cooling system in combination with a tin based plasma radiation
source leads to the further advantage that no contamination will
occur. In addition, no specific redesign regarding material
protection is needed since the used materials are already liquid
tin compatible due to the applied radiation source. Also, in case
of a system malfunctioning or a breakdown, the occurrence of
contamination is practically negligible. As a further advantage, a
liquid tin cooling system can be applied with significant
overpressure, which enables thin channels in the contamination trap
8 without significantly disturbing rotating forces and/or deforming
mechanical parts of the trap 8.
The cooling liquid tin can be collected for re-use, thereby
providing an efficient cooling system. The cooling liquid can e.g.
be collected at a bottom of a chamber in which the contaminant trap
is arranged. Thus, both the cooling liquid and captured Sn debris
emanating from the radiation source can be collected for re-use
purposes in the cooling system.
By arranging the liquid tin cooling system constructed to condition
the temperature of the contaminant trap 10, a solidification
process of tin particles may be counteracted. Solid tin particles
may induce unbalance of the rotating foils 9, and may cause
radiation transmission loss and even failure. Thus, the operation
of the source can be stopped without undesired solidification
process. As an example, the temperature of the supplied liquid tin
can be maintained at a temperature of approximately 250.degree. C.
sufficiently high above the melting point of tin. By maintaining
the temperature of the supplied liquid tin at a pre-determined
degree, the temperature of the contaminant trap 10 may be
conditioned, thereby providing a cooling effect when the
temperature of the trap 10 tends to increase, and providing a
heating effect when the temperature of the trap 10 tends to
decrease below the pre-determined liquid tin supply
temperature.
The radiation system shown in FIGS. 2 and 3 comprises a liquid tin
cooling system wherein a closed liquid tin circuit 12 has been
arranged inside the static part 1 of the contaminant trap 10.
During operation, the closed liquid tin circuit 12 actively cools
the static part 1 of the contaminant trap 10. The liquid tin
cooling system may comprise a semi-open liquid tin circuit
constructed to directly cool a rotating part of the contaminant
trap. The circuit than comprises open channel sections at the
exterior surface of the trap 8. The liquid tin cooling system may
optionally comprise a liquid tin supply channel 13 inside the
static part of the contaminant trap, the supply channel 13
extending to a rotating part of the contaminant trap for supplying
the liquid tin towards an external surface of said rotating part.
The liquid tin cooling system may further comprise a return path
along a leading edge of a foil 9 of the contaminant trap 10. The
liquid tin may create a capillary flow along the leading edge of
the foil, thereby transferring the heat from a segment where the
heat load on the foil is relatively high. The return path may be
embedded in a foil 9 of the contaminant trap 10, e.g. via a semi
open circuit or via interior channels. Due to the geometry and
centrifugal flow, the liquid tin will flow radially outwardly and
drop towards a bottom of the chamber wherein the trap 10 is
arranged.
FIGS. 4 and 5 depict schematic cross sectional views of a
contaminant trap comprised in a radiation system according to
embodiments of the invention. In FIG. 4, the liquid tin cooling
system comprises, apart from the closed liquid tin circuit 12
described above, an exterior supply channel 15 having a spray end
arranged to spray the rotating part of the contaminant trap 10. In
FIG. 5, an exterior supply channel 14 has a spray end that is
arranged near a foil 9 of the contaminant trap 10. Initially, the
cooling liquid tin covers a leading edge of the foil 9 and than
flows over the blades and drops to the bottom of said chamber. It
is further noted that the exterior supply channel 15 is inherently
cooled by the cooling liquid tin flowing through it.
The embodiments described above may provide a reliable tin removal
and effective cooling of the contaminant trap.
In an embodiment, liquid tin regeneration processes, like filtering
and/or chemical cleaning may be performed in the circuit 12 or a
supply channel.
Further, the radiation system might comprise an external heating
system, such as an electrical heating system constructed to enable
the system to start up from a situation in which the tin has been
solidified, e.g. from a maintenance status.
In order to further improve cooling effects of the liquid tin
system and/or radiation transmission characteristics of the
contaminant trap, a contaminant trap exterior surface can be
pre-treated to improve surface wetting characteristics. In an
embodiment of a radiation system 100, as shown in FIG. 6, the
pre-treating step comprises heating said surface. The heating step
is performed by arranging a heating element 104 near the
contamination trap 102. The trap 102 is arranged in the path of the
radiation beam 105 on which the material emanating from the
radiation source 101 can deposit during propagation of the
radiation beam 105 in the radiation system 100. By activating the
heating element 104, the contaminant trap exterior surface is
heated, thereby removing contamination and oxides from its surface.
As a consequence, Sn wetting characteristics of the surface and
thereby also cooling effects are enhanced, since liquid tin will
form a substantially thin coating over the surface. Moreover, the
occurrence of small tin droplets is counteracted, thereby also
improving a radiation transmission of the contamination trap. The
apparatus may further comprise a gas inlet 103 arranged near the
contaminant trap 102. By flowing hydrogen gas in a direction D into
a chamber in which the contamination trap 102 is arranged, the
removal of contamination and oxides from the exterior surface may
be improved. The hydrogen gas may be introduced in the chamber
before the heating element 104 is activated. In addition to, or in
place of applying a separate heating element 104, the radiation
source 101 can be activated at a reduced level to act as a heat
source. After the pre-treating step, the system can be operated
using the liquid tin cooling system.
In an embodiment of the radiation system, shown in FIG. 7, the
system comprises a radical generating unit or a plasma generating
unit 104A that generates hydrogen radicals or a hydrogen plasma,
respectively. By activating a radical generating unit, hydrogen
molecules that are introduced in the chamber are at least partially
transformed into radicals, thereby facilitating the removal of
oxides and contaminants in a faster way and/or at a lower
temperature. The radical generating unit 104A can be implemented as
a hot filament or as a radio frequency discharge element. By
employing a plasma generating unit 104A, contaminations on the
exterior trap surface can be removed. The surface may be treated by
an oxygen plasma.
FIG. 8 depicts a schematic cross sectional view of a section of a
radiation system according to an embodiment. In particular, FIG. 8
shows a central part 106, which may also be called a plug, of the
contaminant trap 8 that is centered with respect to the
longitudinal axis O of the trap. A foil 108 is connected to the
central part 106. The foil 108 may be formed by a material that is
substantially porous. As shown, a liquid tin supply channel 107
ends in the porous structure of the foil 108. During operation, the
liquid tin flows via the supply channel 107 into the porous
structure, via a path I.sub.1, and than via further paths I.sub.2,
I.sub.3, towards the exterior surface of the foil 108. Due to
centrifugal forces of the rotating foil 108 with respect to the
axis O, the liquid tin flows along the surface via paths I.sub.4,
I.sub.5 towards the radial end of the foil 108, thereby uniformly
covering the exterior surface of the foil 108. From the end of the
foil 108, the liquid tin drops from the foil 108 via path I.sub.6
towards a bottom structure of the chamber, where the Sn can be
collected and possibly recycled. By injecting the liquid tin in a
porous structure of the foil 108, a substantially uniform injection
process is obtained, which may provide a relatively smooth liquid
tin layer at the foil surface, which may enhance cooling
properties. In an embodiment, at least a segment of the foil is
substantially porous. The porous segment of the foil may be located
near an end of the liquid tin supply channel 107 and/or near the
exterior surface of the foil 108.
According to a further aspect, an exterior surface of the
contaminant trap, such as an exterior surface of a foil, comprises
a top layer having a low oxidation rate, such as gold. By providing
a top layer having a relatively low oxidation rate, contaminations
and oxides on the exterior surface may be counteracted. As an
example, the foil might be formed by a molybdenum kernel covered
with a thin gold coating. Optionally, the exterior surface has a
low solubility in liquid tin, preferably having a solubility less
than about 0.05%, more preferably less than about 0.005%. Thus, the
foil is not solved during liquid tin cooling processes. Liquid
metals other than Sn may be used. In an embodiment, a Ga--In--Sn,
Ga--Sn, or In--Sn alloy may be used. Such alloys are liquid at a
lower temperature than Sn, which may increase the cooling rate,
reduce a minimum system temperature, and relax the heating
specifications.
FIGS. 9-14 show experimental results of droplets wetting exterior
surfaces of metal plates. In particular, experiments have been
performed simulating the wetting behavior of tin. A Ga--In--Sn
alloy approaches the wetting behavior of tin. Since the Ga--In--Sn
alloy is a liquid at room temperature, the alloy is used for the
experiments. The experiments are performed in an argon atmosphere
to prevent the Ga component in the alloy to oxidize. FIG. 9 shows a
droplet 202 of Ga--In--Sn alloy on a molybdenum (Mo) platelet 201.
Similarly, FIG. 10 shows a droplet 203 of Ga--In--Sn alloy on a Mo
platelet on which a Ga2O3 coating 204 has been deposited. Both
FIGS. 9 and 10 show that hardly any wetting occurs.
FIGS. 11 and 12 show different views of a Ga--In--Sn droplet 301 on
a gold surface 302. As shown, the droplet 301 smears over the
exterior surface 203, thereby providing excellent wetting
properties.
Further, FIGS. 13 and 14 shows a further experiment using a tin
droplet 303. In FIG. 13, the droplet is positioned on a Mo platelet
302 that has been heated in a N2 atmosphere. As shown, poor wetting
characteristics are obtained. However, FIG. 14 shows a similar tin
droplet 305 that has been brought into contact with a Mo platelet
304 that has been pre-treated with hydrogen radicals according to
an aspect of the invention. The droplet 305 now smears again over
the surface 304 providing good wetting characteristics, thus
improving radiation transmission features of the foil.
Although specific reference may be made in this text to the use of
lithographic apparatus in the manufacture of ICs, it should be
understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin-film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "wafer" or "die" herein may be considered as
synonymous with the more general terms "substrate" or "target
portion", respectively. The substrate referred to herein may be
processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may also
refer to a substrate that already contains multiple processed
layers.
Although specific reference may have been made above to the use of
embodiments of the invention in the context of optical lithography,
it will be appreciated that the invention may be used in other
applications, for example imprint lithography, and where the
context allows, is not limited to optical lithography. In imprint
lithography a topography in a patterning device defines the pattern
created on a substrate. The topography of the patterning device may
be pressed into a layer of resist supplied to the substrate
whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
The terms "radiation" and "beam" used herein encompass all types of
electromagnetic radiation, including ultraviolet (UV) radiation
(e.g. having a wavelength of or about 365, 355, 248, 193, 157 or
126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a
wavelength in the range of 5-20 nm), as well as particle beams,
such as ion beams or electron beams.
The term "lens", where the context allows, may refer to any one or
combination of various types of optical components, including
refractive, reflective, magnetic, electromagnetic and electrostatic
optical components.
While specific embodiments of the invention have been described
above, it will be appreciated that the invention may be practiced
otherwise than as described. For example, the invention may take
the form of a computer program containing one or more sequences of
machine-readable instructions describing a method as disclosed
above, or a data storage medium (e.g. semiconductor memory,
magnetic or optical disk) having such a computer program stored
therein.
The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
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